The army of Alexander the Great could be called Macedonian because it fought for the Macedonian king. Its troops were like many other armies in Antiquity only partly recruited from the kingdom itself. These soldiers from Macedonia proper were supplemented by considerable forces from other territories. The native Macedonians however remained the most important part of the army. These men served both in the cavalry as well as in the infantry. The most prestigious of the mounted troops were the hetairoi or companions. The companion cavalry had its origins in the retainers kept by the Macedonian royal house. At first the members of this elite unit were recruited among the Macedonian nobility. During the reign of king Philippus II its strength had however been raised from approximately 600 horsemen to over 3000 troopers. Only part of these were selected among Macedonian nobles, others were recruited from Thessaly and other parts of the Greek world. These hetairoi were organised in ilai or 'wings' of some 200 men except for the basilikè ilè or agèma, the royal squadron, which had a strength of 300 to 400 cavalrymen. In battle these units of Macedonian hetairoi were generally formed up in a wedge formation.

The companion cavalry was equipped with metal helmets and various types of body armour. Some troopers wore linen or leather corselets reinforced with metal scales while others equipped themselves with bronze or iron breastplates. A number of horsemen may even have spurned the use of armour, either for reason of comfort or out of sheer bravado. Shields were probably only reserved for dismounted actions. The hetairoi usually carried a variety of heavy thrusting spears to act as heavy shock cavalry, though they were on occasion armed with javelins. A sword was at all times in use as a secondary weapon. These could be of the hoplite type as well as a curved slashing variety.

Heavy cavalry was very effective against opponents with low morale, but it could do little of consequence when confronted with a determined enemy in good order. Horses are by nature prone to flee any danger, though they can be trained to charge straight at a mass of people at great expense of time and effort. Considering the low resistance of equines to hardships it is however difficult to assess to what extent the Macedonian heavy cavalry could be provided with well trained warhorses. On prolonged campaigns it is very likely many horsemen had to make do with whatever mounts were available.

In addition to the shock troopers of the hetairoi a small number of light cavalrymen designated prodromoi or scouts were part of the native Macedonian cavalry. These horsemen were usually equipped with javelins when employed on reconnaissance missions, but armed with a cavalry version of the sarissa they served as heavy cavalry sarissophoroi in battle. Normally these Macedonians operated closely with the light Paeonian, Illyrian and Thracian cavalry. Confusingly these mounted Thracians were also known as prodromoi.

Beside the cavalry mentioned above there were also infantry units that were recruited in Macedonia. Amongst these the most important were the pezhetairoi or foot companions, of which some were also given the mysterious title of asthetairoi. These pezhetairoi were recruited according to a territorial system in which the various provinces of Macedonia each provided a single taxis or regiment. To reduce the threat of a rebellion at home the army assembled for the Asian campaign was predominantly made up of regiments from the seditious northern districts. Command of the foot companion regiments was usually entrusted to nobles originating from the same area as the men themselves. The various taxeis often carried the name of their commander. Modern studies often assume that each of the foot companion regiments had a strength of some 1500 soldiers, but it is not unlikely that in fact the units as a result of dissimilar losses and replacements had varying numerical strengths. At the start of the campaign six regiments of pezhetairoi were included in the expeditionary army, but during the campaign a seventh taxis was added using reinforcements from Macedonia. It is not known whether this seventh regiment was also territorially recruited or that men from different districts were grouped together.

The pezhetairoi formed the main heavy infantry force of the Macedonian army. The training and armament of these heavy foot soldiers were much more flexible than that of the hoplites in most Greek city states. Equipment and tactics could be adjusted to suit different situations. Armed with the hoplite shield and a spear of normal length the foot companions were capable of deployment in a classical Greek hoplite phalanx. In addition these soldiers could be equipped with a long pike requiring the use of both hands, the famous sarissa, and a different rimless shield hanging from the shoulder to fight in the distinctive Macedonian variant of the phalanx. On a number of occasions these soldiers were also equipped with light javelins instead of spears or pikes. The various sources give different descriptions of the defensive equipment used by the pezhetairoi. It is very probable that this equipment like the offensive weaponry was adapted to suit differing tactical requirements. A helmet appears to have been an item regularly worn by the footcompanions, but additional protection in the guise of metal greaves or a kind of body armour is also mentioned. As worn out armour was burned in India, the body armour was probably constructed of linen, felt or leather. Linen or leather corselets of the same cut as Greek hoplite types are depicted on the socalled Alexander sarcophagus and may have been standard issue. If later Hellenistic practice reflected earlier conditions the officers and NCO's forming the front rank of the formation may have been equipped with metal cuirasses. Body armour of any sort was very uncomfortable in hot weather. On some occasions at least part of the troops may have discarded body armour for this reason. Experiments were also made with armour that offered only frontal protection.

In addition to the pezhetairoi existed an elite formation of hypaspistai or shieldbearers. These men can almost certainly be identified with the socalled argyraspides or silvershields from the later part of Alexander's reign. These soldiers were not recruited on a territorial basis, but selected individually on merit from the taxeis of the pezhetairoi. The hypaspistai numbered three thousand men organised in three subunits of each a thousand soldiers. Although constituting a picked force among the Macedonian infantry one of these battalions, the agema, had a higher prestige than the other two. A modest number of soomatophylakes recruited among the Macedonian nobility was attached to the hypaspistai , which were selected among those of common birth. As these units were not like the taxeis of foot companions depending on replacements originating from a particular district the hypaspistai are likely to have maintained their establishment strength throughout the campaigns of Alexander the Great by constant selection of picked men from the other regiments of Macedonian heavy infantry.

Tactics and equipment of the hypaspistai were probably similar to those of the pezhetairoi, but as an elite formation they were often used for special assignments. In set piece engagements the shieldbearers were generally deployed on the dangerous place of honour at the right flank of the heavy infantry line. Several modern authors assume that these soldiers usually wore lighter equipment in battle than the foot companions, but clear indications for a different armament are absent from the ancient sources. Their frequent use on special duties however meant that the hypaspistai were more likely to carry lighter arms and equipment when not deployed in a set piece battle.

Besides the heavy infantry of foot companions and shieldbearers there was also native Macedonian light infantry enrolled in the army of Alexander. These were made up of javelineers, archers and slingers. Most light infantry was however not recruited in Macedonia proper. The javelin armed Agrianoi stemmed from the neighbouring kingdom of Lagarus. These Agrianoi formed an elite among Alexander's forces and were often employed on dangerous missions. On occasion these troops were used as hamippoi to strengthen the Macedonian cavalry. An additional 7000 strong contingent of Thracians served as peltastai, shieldbearing skirmishers. These Thracians were however not so much selected for their military value, but rather to limit the risk of an uprising in their homelands. The Macedonian archers were supplemented by mercenary bowmen from Crete.

An important contingent in the army of Alexander the Great was the Thessalian cavalry that served the Macedonian king because he was tagos or military leader of Thessalia as well. These horsemen generally operated in battle as the heavy cavalry wing deployed on the left flank of the army. Eight territorially recruited ilai were selected to join the Asian campaign. The Pharsalian ilè had much the same status amongst these squadrons as the royal ilè among the Macedonian companion cavalry. This particular unit may have had a higher establishment strength than the usual two hundred troopers. In contrast to the wedge deployment used by Macedonian and Thracian horsemen the cavalry of Thessaly usually favoured a rhomboid formation. After the war of revenge on the Persian empire was officially brought to an end those Thessalian cavalrymen that opted not to return home were integrated in the reorganised units of the Macedonian hetairoi.

Detachments of forces were also sent by the city states of the Corinthian league to join the retribution campaign against the Persians. These soldiers belonged in part to the professional soldiers of the small standing armies maintaned by the Greek poleis. It is not known to what extent the armament and equipment of these forces varied, but modern literature on the subject assumes that the majority of these troops served as hoplites. The equipment of these hoplites was probably no longer as light as it had been at the turn of the century. Vase paintings and sculptures seem to point to a renewed widespread use of metal body armour by Greek hoplites. In addition to the infantry troops some city states also sent small numbers of horsemen to join the Macedonian army. The troops sent by the Corinthian league had no significant role in any of the battles fought by Alexander. At the end of the official vengeance campaign against the Achaemenid empire these forces were excused further duty. Although the majority of men returned to their cities, some took service as mercenaries in Alexander's army.

Greek mercenaries were also used in the Macedonian expeditionary army. Though these forces were mostly employed for garrison duty in the conquered provinces part of the mercenaries served in the field army. The infantry was composed of both hoplites and peltastai. A number of small mercenary horsemen played an important role in the field army cavalry. Mercenary troops were also hired among the population of the conquered territories of the Persian empire and India. Some of these indiginous forces consisted of mounted javelineers and horse archers, others served as light infantry skirmishers. At the end of Alexander's reign Asiatic troops were levied and equipped and trained on the Macedonian model.

At the head of the Macedonian army chain of command was Alexander the Great himself. The senior officers were partly selected from those that had been brought up with the Macedonian king, though part of the high command consisted of men who had made careers during the reign of Philippus II. Most prominent among the last group was Parmenio, said by Philippus to have been his only general. The relationship between Parmenio and Alexander is traditionally portrayed by the ancient sources to have been plagued by contstant differing of opinions. This image can be corrected by careful reading of the available texts. The course of action advised by Parmenio appears to have adopted by Alexander both before Gaugamela and the battle at the Granicus.

The planning and preparation of battles was of the utmost importance. Because of the very limited means of communicating orders in battle, much depended on the instructions given to subordinate commanders beforehand. The sources indicate the convening of a general staff meeting to discuss plans and preparations. Good reconnaissance and reliable intelligence were vital for proper planning of the engagements ahead. Though a sytem of scouts and spies was employed to furnish Alexander with much needed information, intelligence required the constant personal attention of the commander in chief himself. Before the battle of Gaugamela the Macedonian king went on a reconnaissance foray in person to obtain first hand information.

The officer corps was predominantly structured on territorial divisions. Officers of the various units of hetairoi and pezhetairoi were usually, but not invariably selected from the nobles of the same district as the common soldiers. Junior officers appear to have largely specialised in command of either horse or foot, but senior commanders could be tasked with both infantry and cavalry commands. During the campaigns the importance and prestige of cavalry commands was reinforced. Many officers must have been personally acquainted with their commander in chief through prior service in the paides basilikoi, the royal pages, or the soomatophylakes basilikoi, the royal bodyguards. The selection of senior commanders especially was not always directly related to their military qualities, but was often connected to personal favour, blood ties and political reliability.

The command of an army of many different nationalities posed a number of problems. The various troops spoke different languages and dialects. Officers commanding contingents of foreign forces were probably required to master the use of the Greek language and some rudimentary knowledge of the basic Greek orders may have been thaught to their subordinates. The language barriers were not the only communication problems. Armies in Antiquity lacked effective means of long range communication. Commands by word of mouth had very limited range in the noise of battle. Indications for the use of musical instruments to communicate orders in battle are limited for the army of Alexander. The army also lacked an equivalent of the Roman army standards that could be employed to visually communicate simple orders to the troops. The use of such visual signals was probably very limited anyway because of the enormous clouds of dust raised by the masses of men and horses. Mounted messengers were the main if not fully reliable means of communication on long ranges. Because of these flawed means of communications between the various parts of the battle line the dependance on plans made beforehand and able subordinate commanders was very great. A commander in chief had only effective control over the units in his immediate vicinity and lacked an overall view of the situation. The general staff meetings before an engagement were therefore vital for the coordination of the army's actions.

The battle tactics of the army of Alexander were generally aimed to force a rapid decision. The attack of the Macedonian forces was generally made in an oblique battle formation with an advanced right flank and a refused left wing. A fierce charge of the heavy horse on a small portion of the enemy's forces was intended to break the morale of the enemy and create panic among units not yet engaged in combat. Success depended to a large extent on sapping the morale of an opponent. The use of surprise was an important means to undermine the confidence of the enemy. Unexpected manoeuvres were employed to surprise the opposing forces at the Granicus, Issus and the Hydaspes. It was also important to engage the enemy when his forces were fatigued by long marches or lack of sleep.